As a hollow-type strain wave gearing unit provided with a unit hollow portion passing therethrough in the axial direction, there is known one that is provided with a cup-shaped flexible externally-toothed gear. Patent Document 1 discloses an actuator configured so that a hollow-typo strain wave reduction device and a hollow motor are integrally connected in the axial direction.
In the hollow-type strain wave gearing unit with a cup-shaped flexible externally-toothed gear, a hollow-type wave generator and the annular boss of the flexible externally-toothed gear are disposed adjacent to each other in the axial direction. A bearing holder fixed to the center through hole of the boss is arranged inside the flexible externally-toothed gear. The shaft end portion of the hollow-type wave generator is supported by a bearing held by the bearing holder.
Also, a unit hollow portion extending and passing through the unit in the unit axial direction is defined by the hollow part of the wave generator and the hollow part of the bearing holder fixed to the annular boss. Because of the limitation on the size of the center through hole which can be formed in the boss, the inner diameter of the unit hollow portion is determined by the hollow part of the boss.
Furthermore, in order to prevent a lubricating oil from leaking into the unit hollow portion through the gap between the wave generator and the bearing holder fixed to the boss facing the wave generator, it is necessary to seal the gap. In the actuator described in Patent Document 1, a hollow tube is arranged inside the unit hollow portion, and a sealing mechanism is incorporated between the inner peripheral surface of the wave generator and the outer peripheral surface of the hollow tube.